Cerebral hypoxia or ischemia leads to I. reversible failure of synaptic transmission; II. irreversible loss of neuron function, indicating cell injury; III. delayed deterioration after initial transient recovery following reoxygenation. Previous research in this laboratory has dealt with various aspects of all three phases. The present application is devoted mainly to the mechanism by which neurons lose the capacity to recover from hypoxic insult. The long-range goal of this project is a comprehensive study of the acute effects of hypoxia on central nervous tissue of mammals. Since the objective is the effect of oxygen deprivation itself separated from indirect effects and systemic changes, in vitro preparations are used. In this renewal period we will concentrate on the mechanism and causes of the transition from reversible to irreversible hypoxic change in hippocampal tissue slices. Several hypotheses derived from our previous work will be tested. Recording techniques include conventional microelectrodes, ion selective microelectrodes, and patch-clamping in whole-cell configuration. The proposed experiments will cover the following topics: l. The effects of pH on hypoxic spreading depression (SD)-like depolarization, and on recovery from the effects of hypoxia. 2. Changes of interstitial space during hypoxia, and the role of cell swelling in hypoxic SD-like depolarization. 3. Interaction of hyper-osmotic and hypo-osmotic media with hypoxia. 4. Effects of permeabilizing cell membranes to monovalent cations in hippocampal slices. 5. Membrane changes associated with loss of recovery capacity in CA1 pyramidal neurons in hippocampal tissue slices, investigated with tight seal patch-clamping in whole-cell configuration.